Photovoltaic solar cells convert sunlight directly into electrical energy. Silicon is the dominant material for photovoltaic (PV) solar cells, with over 90% of all solar cells being made from crystalline and polycrystalline Si. In fact, the share of cells made from Si has increased over the last five years, contrary to predictions that bulk Si solar cells would soon be displaced by thin film solar cells. However, periodic shortages of electronic-grade Si represent a serious limitation to the industry. Furthermore the extreme high purity required of Si used in these devices presents major cost, rate, and energy input driver in production of the material. Shortage of Si feedstock contributes to high cost related to solar power generation and, accordingly, much effort has been devoted to research and development into thin film solar cells. Recent progress in manufacturing of CdTe-based devices currently shows prospects for major cost reductions and production scale-up. However, Cd toxicity and ultimately Te mineral resource limitations may prevent this technology from growing beyond a moderate level. Si does not suffer from either toxicity or resource limitations. There is a need for efficient and low cost photovoltaics (PV) made from lower-grade Si feedstock. In addition, processes that reduce Si feedstock waste can also lead to lower-cost photovoltaics and, therefore, lower cost solar energy conversion to electricity.
The most common technology in the PV industry is based on single crystalline and polycrystalline silicon technology. Presently, silicon PV technology has high materials costs, due to the relatively inefficient use of the bulk silicon material. In conventional methods, bulk crystalline silicon is sawn into wafers, which are then processed into solar cells and soldered together to form the final module. Typical multicrystalline efficiencies are on the order of 15%; high-performance, single-crystal silicon has been produced with 20% efficiency. For this type of solar cell, 57% of the cost is in materials, and of that total material cost 42% comes from the crystalline Si. In addition, these modules are rigid and heavy, both of which are negatives from the consumer standpoint.
Reviews of amorphous and crystalline thin-film Si solar cell activities suggest that near term prospects for significant improvements in thin film amorphous Si cell efficiencies and stability are unlikely without an unexpected breakthrough, thus limiting producers to low margin, highly competitive market applications. It was recognized also that crystalline-Si (c-Si) technologies are achieving economies of scale that allow them to be cost competitive with thin film Si in spite of the energy intensive processes to form c-Si and the large device thicknesses (>100 μm). As the market continues to grow from megawatt to gigawatt scale, however, such thick devices are expected to lose this cost advantage and thin film devices will become the logical choice. An example of this change was provided recently when a thin film CdTe manufacturer announced a production cost under $1.25 per rated Watt of generating capacity with prospects for reduction to below $1/W. This product is undergoing rapid scale-up and may ultimately exceed the production volume of c-Si. Therefore it is important that c-Si make further improvements in cost to remain competitive with the thin film products.
It has been suggested that the greatest opportunity for significant improvements in thin film Si solar cell efficiency, affordability, and stability are likely to occur through crystalline thin-film Si technologies. Both low temperature processable nano/microcrystalline-Si (n/μc-Si) and higher temperature processable small grain polycrystalline-Si (pc-Si) are receiving much attention worldwide as a potential replacement for amorphous-Si (a-Si) alloys used in single junction devices and as the bottom cell in multi-junction devices. While early successes demonstrated near state-of-the-art thin film Si cell efficiencies, which were stable for long periods of time, the ultimate potential of crystalline thin film Si technologies remains less clear due to their higher processing cost and slower throughput when compared to amorphous Si. Unlike crystalline thin film Si, analysis has shown that the cost structure of CdTe and CIGS can be favorable but that issues related to material availability, low efficiency, and Environmental, Health, and Safety present considerable market risk.
Although cells that incorporate “slivers” of monocrystalline Si (i.e. the SLIVER technology) have some apparent connection to the processes provided herein, there are many key differences. First, SLIVER uses Si thicknesses that are relatively thick, typically in the range of ˜50 μm. Not only does this require more input Si material, but it also requires better material because the collection field of the solar cell junction is more distributed and carriers must move farther. The proposed technology can provide equivalent cell performances in lower purity Si material due to the reduced device thickness. Second, various approaches presented herein exploit a unique printing based assembly process that is important for achieving cost effective manufacturing with thin Si. Third, current embodiments of SLIVER modules are not mechanically flexible, and they cannot readily incorporate other key technologies, such as molded low concentration micro-optics, and printed interconnects. Micro-optics can increase device performance in otherwise equivalent devices and materials due to the performance enhancement that the concentrator design provides. These multiple differences yield qualitatively different types of modules, with different performance and cost characteristics.
It will be appreciated from the foregoing that a need exists for methods of making high performance solar cells from cheaper forms of Si, such as low-grade Si and Si wafers. To further reduce production costs, solar cell production methods are needed that are capable of high-throughput, low cost implementation with minimal waste of Si feedstock. Further, there is a need for photovoltaics having good operating characteristics and enhanced mechanical functionality such as flexibility, bendability and that are lightweight to promote shipping, handling and installation ease.